1. Field of the Invention
The present invention is generally directed to methods and devices for mining, and, more particularly, to methods and devices for rotary mining.
2. Description of the Related Art
Several conventional mining techniques can be employed to remove subterranean material. Such techniques commonly utilize machinery adapted to remove coal, for example, from seams that are relatively deep beneath the surface and require a network of mines comprising underground shafts and passages to access the seams. Such machinery is used to loosen material from the seams and transport the material to the surface; however, personnel are required to enter the mines to operate the machinery thereby placing them in dangerous underground conditions. Another mining technique, commonly referred to as surface, or strip, mining, is used to remove material that is relatively close to the surface. In strip mining, overlying dirt, rocks, and gravel, i.e., overburden, is removed from the ground to expose a coal seam, for example. However, strip mining often requires the use of expensive machinery to remove the overburden and often has an adverse environmental impact on the area being mined.
Other mining techniques and devices have been recently developed which solve many of the above-described problems. U.S. Pat. No. 6,065,551, for example, discloses such methods and devices. In one exemplary embodiment, a rotary mining device having radially extendable cutting members is inserted into a subterranean shaft, or bore hole, to loosen material from the sidewalls of the shaft. In such embodiments, a coal seam can be comminuted into powder, drawn up the shaft and collected when it reaches the surface. As a result, the expense of developing a network of underground passages is obviated and the surrounding environment can be substantially preserved. As disclosed therein, the cutting members are radially extended and retracted with respect to the mining device as a result of centrifugal force acting on the cutting members when the mining device is rotated. More particularly, as the rotational speed of the mining device is increased, the centrifugal force acting on the cutting members is also increased and, as a result, the cutting devices are extended further away from the mining device. Similarly, as the rotational speed on the mining device is decreased, the centrifugal force acting on the cutting members is also decreased and, as a result, springs within the mining device can retract the cutting members. Although such devices are quite successful for achieving their intended purpose, the speed of the mining device and the distance which the cutting members are extended from the mining device are directly, and indivisibly, related. As a result, the operating conditions of the mining device can be somewhat limited which can, in some circumstances, decrease the efficiency and, thus, the profitability of the mining device. What is needed is an improvement over the foregoing.